The paper is engaged in a study of the crane structure safety assessment method based on the unascertained measuring theory. With the rapid economic development of modern society, more and more widely use of various cranes are found in the construction and transportation areas, particularly, in the construction area. With the increase of the national infrastructure investment, much greater varieties of crane demands have been brought about to the crane manufacturers. However, higher and higher standards have been established to the safety operations of crane installations and workers’ operation skills so as to protect people and material properties from any kind of unexpected accidents in addition to the social stability. It is just from this increasing safety need that we have worked out a safety evaluation model by using the unascertained mathematical theory, which is featured as being simple, practical and reliable. The safety evaluation model we proposed for the overhead crane can be divided into four stages, by using the assessment indexes as unascertained information and the confidence level. Taking 5 overhead cranes as examples, by identifying a single indicator matrix and unascertained measure weights, we have formulated a multi-attribute integrated measurement matrix system with the sample being identified and sorted out. The results of our assessment practice prove to be in nice conformity with the results of the software analysis results. The model initiated by us can not only help to evaluate a single crane security, but also the cranes with roughly equal security situations. Case studies also show that the use of unascertained measure model is reasonable in theory and credible in results. It not only includes ordering of information uncertainty but also the evaluation of the space, so that the evaluation result can be made rational, sophisticated and practical.